1. Field of the Invention
This invention relates to a transient protector, particularly for protection of electronic circuits from lightning strikes and, more specifically, to a system for protecting equipment at a telephone central station from damage due to lightning strikes on the external telephone lines.
2. Brief Description of the Prior Art
Devices and circuits for protection from lightning strikes and particularly such circuits for protection against lightning strikes in electronic circuits are well known in the prior art. It is known that lightning strikes can be of either positive or negative polarity relative to the reference voltage of the protected circuit. It is therefore necessary that any protection circuit be equipped to handle both positive and negative lightning strikes.
The above noted problem with lightning is especially acute in the case of telephone circuits and particularly such circuits at telephone central stations. In such circuits, the subscriber is connected to the central station, often through many miles of telephone line or cable. Lightning is often attracted to the exposed telephone lines and causes a transient, positive or negative, to travel down the line to circuits at the central station, such as, for example, the subscriber line interface circuit (SLIC), wherein such transients can burn out semiconductor circuit components therein. SLIC circuits generally include a ground line, a tip line and a ring line, both the tip and ring lines generally being negative with respect to the ground line. Circuits are located between the ground line and the tip and/or ring lines. It is therefore readily apparent that transients caused by a lightning strike can and often do travel through such circuits and cause burn out therein.
In the prior art, the above noted lightning problem has been handled by, for example, the use of a reference voltage controlled trip circuits, one such trip circuit coupled between the tip line and ground the the other trip circuit being coupled between the ring line and ground. Each trip circuit controls a separate SCR having its anode coupled to ground and its cathode coupled to the tip or ring line. In addition, diodes were coupled across the tip line and ground and across the ring line and ground with the cathode of each diode at ground. In circuits of this type, current from positive-going transients due to lightning travelled through the diode to ground whereas negative-going transients caused an increase in current travelling to the trip circuit, thereby causing the SCR to conduct when a predetermined threshold was reached, this threshold generally being the battery voltage at the central station.
Circuits of the above described type perform the function for which they are designed. However, in order to obtain semiconductor diodes capable of passing current of the magnitudes required without burning out themselves, it has been necessary that these diodes be very large and occupy a large area of the semiconductor chip. It therefore follows that chip packing density is sacrificed, this being an important consideration in semiconductor circuit design. Accordingly, it is desirable to provide circuit designs which lend themselves to greater component packing density. In addition, due to the large amounts of current potentially being drawn by such protective circuits, hot spots developed at regions of high current flow due to the voltage drop created in the diode. It is highly desirable that such hot spots be eliminated or at least minimized.
Another problem encountered with the prior art circuits is that, when the tip or ring line goes negative with respect to Vs, the central office battery voltage which is negative with respect to ground, an abnormal situation since Vs is the most negative voltage normally in the circuit, trip current will be drawn. This causes a reverse current to be drawn in the central office battery. In the prior art, a filter, such as a capacitor, has been used to support that current during transient and prevent large currents from travelling through the lateral transistor at the trip circuit. It would be highly desirable to minimize the current drawn from the battery to minimize the risk of overcurrents in the trip circuit lateral transistor, thereby minimizing and possibly eliminating the need for the filter.